Transformer having tapped section between interconnected static plates



Nov. 8, 1955 F. J. VOGEL TRANSFORMER HAVING TAPPED SECTION BETWEEN INTERCONNECTED STATIC PLATES Filed July 10, 1953 lllLv Patented Nov. 8, 1955 ice TRANSFORMER HAVING TAPPED SECTION BE TWEEN IWTERCONNECTED STATIC PLATES Fred J. Vogel, Wauwatosa, Wis., assignor to Allis-Chalmers Manufacturing Company, Milwaukee, Wis.

Application July 10, 1953, Serial No. 367,149

3 Claims. (Cl. 336-70) This invention relates generally to transformers of power distribution systems and particularly to such apparatus having an autotransformer winding with a variable low voltage connection. 1

Power transformers are designed to withstand voltage surges and preferably utilize the so-called shell type design with the winding stack comprised of superposed disk coils formed of turns of strand that are spirally wound. The disk coils are connected in series circuit relation, start to start and finish to finish. An advantage of the shell design over the core type design is the more uniform distribution of a voltage surge through the turns of the winding.

When the power transformer is an autotransforrner having an intermediate tapped winding section for a variable low voltage lead, cost and design complications arise. One reason is that surge voltages may be applied to any tap to which the low voltage lead connects. While electrostatic shields have been disposed adjacent the turns near terminals of power transformers including autotransform: ers, the prior suggested application of such electrostatic shields to an autotransformer is to dispose an electrostatic shield adjacent the particular coil to which the low voltage lead is tapped. But to provide such shields in an autotransformer having a plurality of low voltage taps would considerably increase the insulation requirements, increase the axial length of the winding stackand increase the size and cost of the step regulator transformer.

An object of thisinvention, therefore, is to provide an autotransformer having a plurality of low voltage connections of minimum size and cost.

Another object of this invention is .to reduce the turn to turn stress in the coils of the tapped portion of an autotransformer provided with a plurality of low voltage connection-s.

And another object of this invention is to reducethe spacing and amount of insulation required between disk coils in the tapped section of an autotransformer provided with a plurality of low voltage connections.

Still another object of this invention is to reduce the voltage between a plurality of low voltage tap leads of a transformer when the low voltage terminal is subjected to surges, whereby the size and cost of tap changer mechanism may be reduced.

Still another object of this invention is to provide an autotransformer having a plurality of low voltage taps in which the tapped section of the autotransformer winding is confined to the minimumpercentage of the axial length of the winding stack and is entirely disposed within an electrostatic field of substantially uniform potential.

And still another object of this invention is to provide an autotransformer having a tapped low voltage winding section entirely disposed in an electrostatic field responsive to voltage surges imposed on the low voltage terminal regardless of the tap to which that terminal is connected whereby the surge does not impose damaging turn to turn stresses in any part of the tapped winding section.

Objects and advantages other than those above set forth will be apparent from the following description when read in connection with the accompanying drawings, in which:

Fig. 1 is an elevation view of a shell type transformer in which this invention may be embodied;

Fig. 2 is a plan view of the transformer shown in Fig. 1;

Fig. 3 is a schematic diagram of an autotransformer winding and associated tap changer to which this invention relates and represented as taken on line lIIIlI of Fig. 2. The insulation is omitted in order to clarify the showing of the winding; and

Fig. 4 is a curve showing the turn to turn stress of the winding illustrated in Fig. 3 when subjected to impulse test.

The winding leg of the transformer iron core is designated by the numeral 11. The transformer is of the shell type design comprising winding stack 12, encircling the winding leg of the iron core with suitable insulation such v as conventional tubing and padding between the iron core finish to finish with a different other coil.

and winding. The transformer is represented as being in an upright position and for convenience the opposite ends of the winding stack will be referred to as top and bottom, but, of course, the vertical positioning of the transformer has no bearing on this invention.

Winding stack 12 comprises a plurality of disk coils of strand spirally wound from the start at an inner end thereof the finish at the outer extremity thereof. These coils are disposed about the winding leg by being superposed over each other, and they have a common axis at the center of the winding leg. The sequence of disposition of these coils on the winding leg may be varied depending upon the conditions for which the transformer is designed. The coils are connected in series circuit relation, and except for the first and last of the series, each coil is connected start to start with one other coil and A neutral or ground wire 20 is connected to the finish of coil 21, the first of the series of coils which is connected start to start with coil 22.

From coil 22 through coil 39 each of the remaining coils of the illustrated winding stack is connected start to start to the coil adjacent thereto on one side and finish to finish to the coil adjacent thereto on the opposite side. And coil 39 is connected start to start with coil 40, the last in the series, which has its finish connected to high voltage terminal 53.

While any desired number of coils and taps may be provided for the tapped section, only three taps are illustrated, and four coils 29, 30, 31 and 32, comprise the tapped section of the winding. Low voltage terminal 51' may be selectively connected by tap changer 52 to any tap at these coils. This tapped section is disposed between two portions of the non-tapped section which includes all the series connected coils mentioned above except of course the four in the tapped section. One of those nontapped portions comprises the eight coils 21 to 28 at the upper end of the stack and the other portion comprises the eight coils, 33 to 40, below the tapped section.

The insulation distances between the inner peripheral. edges of the coils and the iron winding leg progressively increases in thickness from one end of the series winding to the other end thereof as the potential in those cells increases. And the spacing and insulation between adjacent coils is varied generally proportional to the number of turns between the respective adjacent coils. The interconnected ends of adjacent coils are at the same potential whether those interconnections be start to start or finish to finish and are therefore shown at a minimum spacing determined by thecooling and mechanical requirements. But the noninterconnccted ends of any two adjacent coils, being at potential difi'erences determined by the number of turns in series in the adjacent coils, are spaced as determined by the voltage stress requirements.

The number of turns in each coil of the tapped section is determined by the tapped voltages desired. As is well known, the taps cannot be connected to an intermediate turn in a coil because of the mechanical stress that would result under abnormal current conditions, but must be connected either at the start or finish ends of coils. Therefore, the number of turns spirally wound in any coil of the nontapped section of the winding is relatively many as compared to the number of turns spirally wound in any coil of the tapped section. This is so because the number of turns in the coils of the nontapped section is limited substantially only by insulation requirements and the desired transformer characteristics; whereas the number of turns in a coil of the tapped section is limited by the percentage regulating steps and the necessity of each tap being connected to the start or finish extremity of coils. Whereas in the illustrated embodiment, there may be as few as fifty-seven turns in each coil of the first portion and as many as seventy-two turns in each coil of the second portion (the thickness of wire may vary), there may be only from eleven to twenty-five turns in a coil of the tapped section. Tap 61 is connected to the finish to finish interconnection between coils 28 and 29, tap 62 is connected to the finish to finish interconnection between coils 30 and 31, and tap 63 is connected to the finish to finish interconnection between coils 32 and 33.

Any of the three taps 61, 62 or 63 may be connected to low voltage terminal 51, and the transformer is designed so as not to fail when a lightning surge is impressed on that terminal. In accordance with the teaching of this invention the turn to turn stress in the coils of the tapped section, the spacing between those coils, and the cost and size of the entire transformer are kept to a minimum by physically disposing the entire tapped section within an electrostatic field of relatively uniform potential.

Electrostatic shields 66 and 67 shaped as disks as are the coils are disposed at the opposite ends of the tapped section. These shields are interconnected by lead 68 to which low voltage terminal 51 is attached at 69 by passing tap changer 52. Regardless of the number of low voltage taps in the autotransformer winding, only two electrostatic shields need be associated with the tapped section. And since these two electrostatic shields are disposed so that the entire tapped section is therebetween and they are connected to the low voltage lead independently of the taps and tap changer, there is no great electrostatic potential difference across the tap section. That potential corresponds to the potential of the low voltage terminal.

A surge voltage imposed on that terminal is therefore impressed equally on all turns of the tapped section to ground. That is, there is relatively little potential across the turns of the tapped section.

As there is no great turn to turn stress in the tapped section resulting from a surge imposed on the low voltage terminal, the tap changer size may be relatively small because the voltage between low voltage leads likewise will be substantially uninfluenced by the surge.

Further, with the tapped section disposed intermediate the two portions of the nontapped section, those static plates disposed at opposite ends of the tapped section are each also disposed adjacent a coil at the end of one of the nontapped portions, whereby surges impressed on terminal 51 are distributed through the turns of an end coil of each of the two portions of the nontapped section.

Other features of the winding illustrated are conventional. An electrostatic shield 71 is connected by lead 72 to the high voltage terminal and is disposed adjacent coil 40, to which that terminal is connected.

In Fig. 4 the solid line is the curve of the turn to turn stress in the winding of the autotransformer described above when subjected to a surge test. The percent winding or number of turns in the winding is represented along the abscissa and the percent maximum surge voltage is represented along the ordinate. The surge was applied at point 81 corresponding to tap 63. Point 82 represents tap 61. There are seventy-three turns in the four coils of the tapped section disposed between electrostatic plates 66 and 67 in an electrostatic field where there is no great stress between those turns. And on opposite sides of the tapped section, the two nontapped portions are shown to have relatively characteristic turn to turn stress distribution.

The dotted line in Fig. 4 represents the turn to turn stress distribution when the tapped portion of the winding is not within the floating equipotential electrostatic field in accordance with this invention but instead an electrostatic shield is disposed adjacent to and connected to each coil to which the low voltage lead may be connected. Such a shield would impose the stress on the relatively few turns of the coil immediately adjacent thereto, and the turn to turn stress throughout the tapped portion of the winding would be quite great, as can be seen by the curve, requiring considerably greater spacing and insulation than for an autotransformer made in accordance with this invention.

Although but one embodiment of this invention has been illustrated and described, it will be apparent to one skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. A transformer comprising a magnetic core including a winding leg, an electrical winding comprised of disk type coils of strand wound spirally, said coils superposed over each other encircling said leg and spaced axially there along, conductors connecting adjacent said coils in series relation alternately start to start and finish to finish, a first plurality of more than two of said coils constituting a tapped section, a second plurality of said coils divided into two portions with said tapped section disposed inter mediate said two portions, a plurality of more than two tap leads connected to said first plurality of coils, a terminal, means for selectively connecting said terminal to any of said tapped leads, two electrostatic shields, said first plurality of coils being disposed between only said two electrostatic shields, and conductive means electrically connecting said shields and said terminal to place said first plurality of coils in a substantially uniform electrostatic field of the potential of said terminal regardless of the tap to which said terminal is connected.

2. A transformer comprising a magnetic core including a winding leg, an electrical winding comprised of disk type coils of strand wound spirally, said coils superposed over each other encircling said leg and spaced axially therealong, conductors connecting adjacent said coils in series relation alternately start to start and finish to finish, a first plurality of more than two of said coils constituting a tapped section, a second plurality of said coils divided into two portions with said tapped section disposed intermediate said two portions, three tap leads connected to the interconnections between adjacent coils of said tapped section, said tapped section being between only two electrostatic shields disposed axially apart on said winding leg, a terminal, means for selectively connecting said terminal to any of said tap leads, and conductive means electrically interconnecting said two shields and said terminal.

3. A transformer comprising a magnetic core including a winding leg, an electrical winding on said leg comprised of disk type coils of strand wound spirally from an inner end defined as the start to an outer end defined as the finish, said coils superposed over each other and spaced axially along said leg, electrical means connecting adjacent said coils in series alternately start to start and finish to finish, said winding divided into a first plurality of more that two of said coils constituting a tapped section and a second plurality of coils divided into first and second portions, said tapped section disposed between said portions, each of the coils of said tapped section having relatively few turns of strand spirally wound, each of said 2,728,379 5 6 second plurality of coils having relatively many turns of means connecting said electrostatic shields to each other strands spirally wound, said first plurality of coils being and to said terminal. disposed between only two electrostatic shields with a first said electrostatic shield disposed at one end of said tapped References Cited in the file of this Patfimi section between said tapped section and said first portion 5 UNITED STATES PATENTS and a second said electrostatic shield disposed at the other end of said tapped section between said tapped section igggigi $332 2 and sald second portion, a terminal, means for selectively 1,872,293 Hodnette Aug. 16 1932 connecting said terminal to the interconnection between adjacent said coils of said tapped section, and circuit 10 

